1. Field of the Invention
The present invention relates to an apparatus and method for setting a frequency of an oscillator with an oscillator circuit including at least one oscillator circuit inductor and a first oscillator circuit capacitor, whereby the value of the first oscillator circuit capacitor is reversible by first control voltages between different stages.
2. Description of the Background Art
An oscillator whose frequency is controlled by an analog tuning voltage in conjunction with a pair of varactor diodes is known from the article “A Fully Integrated SiGe Bipolar 2.4 GHz Bluetooth Voltage Controlled Oscillator”, 2000 IEEE Radio Frequency Integrated Circuits Symposium.
Such oscillator circuits are used, for example, in voltage controlled oscillators and in phase locked loops to generate a signal of a defined frequency. They often have a resonator (oscillator circuit) including a parallel connection of inductive and capacitive components. In such an oscillator circuit, high currents resonate back and forth between the inductor and capacitor at the resonance frequency, whereby only small currents flow in the supply lines. Attenuation losses are compensated by an external connection, which in an ideal case, supplies radiated in-phase and/or Joule loss energy to the oscillator circuit.
Instead of a requirement for a sharply defined, fixed resonance frequency, in technical applications a certain bandwidth is usually required, within which the resonance frequency of the oscillator circuit is to be adjustably controlled. In Bluetooth applications (radio communication over short ranges on the order of several meters), this is, for example, a bandwidth of 0.1 GHz between 2.4 and 2.5 GHz. The desired frequency is usually set by a controlled change in the oscillator circuit capacitor. This oscillator circuit capacitor is often realized by capacitance diodes, which have a control voltage-dependent capacitor.
In addition to this desired dependence, which is utilized for control, unwanted dependencies also occur, which are caused, for example, by fabrication-related variations in component properties and by fluctuating operating temperatures. For this reason, the setting range of the control must also cover the frequency offset due to temperature drifts and fabrication tolerances. In this case, the setting range for the control is determined by the properties of the employed components, for example, by the possible capacitance variation of capacitance diodes and by the circuit properties, particularly by a maximum range of possible control voltages. In mobile communications technology, for example, supply voltages between 3 V and 3.6 V are typical, which greatly limits the possible capacitance variation.
In conventional oscillators, the entire oscillator circuit capacitance is divided into parallel, switchable oscillator circuit capacitors and a capacitor that can be varied via an analog control voltage V_tune. The parallel switchable individual capacitors are connected in a controlled manner to the oscillator circuit or disconnected from the oscillator circuit. This solution has the disadvantage of a high space requirement, because this increases with the number of the capacitors to be connected. In addition, the prior-art circuit does not resolve the problem of temperature dependence or at least resolves it inadequately. A phase noise is always increased when the frequency range selected and to be tuned by means of V_tune is larger than required by a pure tuning range. This is the case when either all capacitors are controlled analogously by V_tune; or the capacitors controlled by V_tune, in addition to the actual tuning range, also experience fabrication tolerance-related and/or temperature drift-induced frequency detuning.